This invention relates to a differential gear device for use in a motor vehicle. More particularly, it relates to two aspects of the differential gearxe2x80x94the control of input torque to the device and the control of output torque from the device.
Differential gear devices, commonly referred to as differentials, are well known devices used in motor vehicle drive lines. A differential gear is designed to distribute torque from an input element to two output elements while permitting the two output elements to rotate at different speeds under certain conditions. The output elements may be connected to two wheels alongside one another at opposite sides of, a vehicle, in which case the wheels are required to rotate at different speeds when the vehicle is cornering. The differential may alternatively be an inter-axle differential in a multiple-wheel-drive system, in which case the wheels on the two axles may be required to rotate at different speeds from one another again, for example, when cornering.
In two-wheel drive vehicles, it may be desirable to have the facility to switch to a four-wheel drive system if the vehicle needs more traction. It is known to provide an auxiliary drive to the second axle of the vehicle, in the form of an electric motor drive, to provide this four-wheel drive system. The engagement of such a drive may be achieved by a controlled clutch which engages the auxiliary drive to the second axle.
It is also known to provide a differential with a means for inhibiting the differential action thereof. The differential action may be inhibited to the extent that it is completely locked, i.e., no relative rotation is possible between its two outputs. This is carried out to overcome problems with uneven traction surfaces such as where a wheel or wheels driven by one of the differential outputs is on a slippery surface and the wheel or wheels driven by the other of the outputs is on a surface which is not slippery. Under these conditions, a differential without any means for inhibiting or locking its differential action directs only a small torque to both wheels, limited by that torque transmitted by the slipping wheel thus potentially immobilizing the vehicle.
On existing systems which incorporate both an auxiliary drive to the second axle and a differential inhibiting means, the method of engaging these two systems would be achieved by two separate clutch systems. This method is very expensive to install, and also the engagement times for the systems are not fast enough to react to emergency situations such as when the driver is no longer in control of the motor vehicle.
The present invention provides an auxiliary drive allowing a two-wheel drive vehicle to be temporarily converted into a four-wheel drive vehicle, while also inhibiting the differential action of the differential on the auxiliary drive to cope with the aforementioned problems on uneven traction surfaces.
According to one embodiment of the invention, a differential is provided comprising: a driving member; an input member; first and second rotatable output members; differential gearing operable between the input member and the first and second output members, for transmitting rotation from the input member to the first and second output members and providing for differential rotation of the first and second output members relative to one another; an engaging device operable to establish a driving connection between the driving member and the input member; an inhibiting device operable to inhibit relative rotation between the first and second output members; and an actuating device for causing the operation of the engaging device and the inhibiting device.
In another embodiment, the actuating device is adapted to cause sequential operation of the engaging device and the inhibiting device.
In a further embodiment, the engaging device is operated to establish the driving connection between the driving member and the input member prior to the inhibiting device being operated ultimately to lock the first and second rotatable output members so that there is no relative motion therebetween.
The engaging device and the inhibiting device may be contained within the input member. The inhibiting device may be a first clutch mechanism such as a multi-plate clutch pack. The engaging device may be a second clutch mechanism such as a multi-plate clutch pack.
In a further embodiment, the actuating device comprises an electric rotational actuator such as an electric motor, and an actuator member operable on the engaging device.
In order for the engaging device to be operated prior to the inhibiting device, the operation of the inhibiting device by the actuating device may be effected through a first spring, exerting a force which has to be overcome before the inhibiting device is operated. In one example, the engaging device is operated by the exertion of a force thereon by the actuator, which force is reacted against the first spring.
In another embodiment, the force to operate the engaging device is exerted through a second spring. The inhibiting device is operated by the exertion of a force thereon by the engaging device through the first spring and against the input member.
When the operation of the actuator member is partially reversed, the first spring may return the inhibiting device to its original position, thereby allowing the first and second output members to rotate relative to one another.
When the operation of the actuator member is completely reversed to its original position, the second spring may return the engaging device to its original condition, thereby disconnecting the driving connection between the driving member and the input member.
The first spring can have a higher stiffness than the second spring, thereby allowing the engaging device to establish a driving connection between the driving member and the input member with enough force to transmit torque from the driving member to the input member, without engaging the inhibiting device.
To allow smooth and sequential engagement of the engaging device and the inhibiting device, the actuating device may be a ball ramp actuator.
In another aspect of the invention, the drive to the driving member is an electric motor.
Other advantages and features of the invention will also become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.